When mobile nodes such as mobile telephones and personal computers move between each network on an IP network, as a technology that achieves mobility by automating the management of IP addresses and the transfer of communication packets to the destination, there is Mobile IP, standardized by Internet Engineering Task Force (IETF).
FIG. 13 is a schematic diagram for explaining an environment in which the Mobile IP is used. According to this Mobile IP, as shown in FIG. 13, by registering an association between a care-of address (CoA) assigned to the network interface on an external network to which a mobile node 1 connects at its mobility destination and a home address (HoA), which is an address on a home network where the mobile node 1 belongs, to a home agent 2, continuous communication with a communication node 3 can be realized via a network 4 even when a link that the mobile node 1 connects to changes. For instance, when the communication node 3 transmits a packet whose destination is HoA, since the home network is a network where the home agent 2 belongs and the home agent 2 responds with its own link-layer address in a case where a link-layer address for HoA as a proxy of the mobile node 1 is needed while the mobile node 1 exists on the external network, the packet to HoA transmitted by the communication node 3 is sent to the home agent 2 by general routing processing. When the packet to HoA reaches the home agent 2, based on the association information between HoA and CoA that it holds, the home agent 2 encapsulates the packet with the address of the home agent 2 HA_addr as a source address and the address of the mobile node 1 (CoA) as a destination address, and transmits the packet to the mobile node 1.
Further, when the mobile node 1 sends a packet to the communication node 3, the packet will be canceled by an ingress filter if the source address does not belong to the network address in which the mobile node 1 exists. Because of this, the packet having HoA (the address of the mobile node 1) as the source address and the address of the communication node 3 (CN_addr) as the destination address is encapsulated as a packet having CoA (the address of the mobile node 1) as the source address and the address of the home agent 2 (HA_addr) as the destination address and transmitted to the home agent 2 for time being. After releasing the capsulation of the packet, the home agent 2 transfers the packet to the communication node 3. Further, when the network to which the mobile node 1 connects changes thereby changing CoA, the mobile node 1 transmits a registration request for updating the association between HoA and CoA each time. The home agent 2 updates CoA associated with HoA according to the registration request. By the processing described above, continuous communication between the mobile node 1 and the communication node 3 is achieved.
However, in a system in which one single mobility anchor such as the home agent 2 in the mobile IP described above is intensively responsible for transferring packets, the load is concentrated on the mobility anchor. In such a case, the load can be distributed by providing a plurality of identical mobility anchors.
Conventionally, for instance Patent Document 1 describes such a technology.
In a load balance method disclosed in Patent Document 1, communication load information is exchanged between a plurality of home agents on a regular basis, and when one of the home agents has more load than a predetermined amount, the load is distributed by having a mobile node transmit a registration request to another home agent with a smaller load.
Further, regarding a home agent device described in Patent Document 2, on an IP network where a plurality of home agent devices are provided, each home agent device measures the load information of a BC (binding cache), and when a home agent device requests another home agent device to transfer a BC, the home agent device that has received the request judges whether or not it accepts the BC based on the BC load on the device at the time and on the load of the BC transferred, and the loads are distributed based on the result of the judgment.
Further, in a mobile communication system described in Patent Document 3, the load status of packet processing devices is monitored by a packet processing representative device, and how communication to/from a mobile body is established is specified based on the current load status. As a result, the loads on the packet processing devices are distributed and the operation efficiency of the system can be improved.    [Patent Document 1]    Japanese Patent Kokai Publication No. JP-P2005-027315A (Abstract, FIG. 2)    [Patent Document 2]    WO2004-105329A1 (Abstract, FIG. 2)    [Patent Document 3]    Japanese Patent Kokai Publication No. JP-P2004-207851A (Abstract, FIG. 2)